1. Field of the Invention
The present invention relates to an amplifying device and converter thereof. More particularly, the present invention relates to an amplifying device and converter thereof having a rail-to-rail input and a substantially constant transconductance.
2. Description of the Related Art
A conventional operational amplifier can be used to amplify the difference between two input signals. In general, the output voltage of a voltage operational amplifier is equal to the difference between the non-inverted input voltage and the inverted input voltage with multiplication by the voltage gain of the operational amplifier. For an ideal voltage operational amplifier, the input impedance is relatively high and the output impedance approaches zero. On the other hand, the output current of an operational transconductance amplifier (OTA) is equal to the difference between the non-inverted input voltage and the inverted input voltage with multiplication by the transconductance gain of the operational amplifier. For an ideal operational amplifier, both the input impedance and the output impedance are relatively high.
To obtain an operational transconductance operation amplifier with rail-to-rail input using the conventional technique (that is, the output voltage range is quite close to the voltage range of the input power), the input range has to be expanded. FIG. 1 is a circuit diagram of a conventional operational transconductance amplifier. As shown in FIG. 1, the operational transconductance amplifier 100 is formed by connecting a P-channel operational transconductance amplifier 102 and an N-channel operational transconductance amplifier 104 together. The non-inverted input terminal (the non-inverted signal being V+) of the operational transconductance amplifier 100 is connected to the non-inverted input terminals of the operational amplifiers 102 and 104. The inverted input terminal (an inverted signal being V−) of the operational transconductance amplifier 100 is connected to the inverted input terminals of the operational amplifiers 102 and 104. The output terminal of the operational transconductance amplifier 100 is connected to an output terminal Iout.
As shown in FIG. 1, when the common mode signal Vcom (defined as the average value between V+ and V−) of the input signals V+ and V− is high enough to approach the highest voltage of the input power source of the operational transconductance amplifier 100, the P-channel operational transconductance amplifier 102 is shut down and yet the N-channel operational transconductance amplifier 104 can still be in operation. Contrarily, when the common mode signal Vcom is low enough to approach the lowest voltage of the input power source of the operational transconductance amplifier 100, the N-channel operational transconductance amplifier 104 is shut down and yet the P-channel operational transconductance amplifier 102 can still be in operation. Therefore, the input range of the operational transconductance amplifier 100 is larger than that of the operational amplifier 102 or the operational amplifier 104.
However, in addition to expanding the input range of the operational amplifier, the transconductance of a conventional operational amplifier must be maintained to have a substantially constant value. Because of this, there is a need to improve the operational transconductance amplifier in FIG. 1 so that a substantially constant transconductance is obtained. Aside from improving the design of a conventional operational amplifier, it is important to reduce the complexity of circuit design and minimize the layout area of devices so that the operational amplifier can be produced at a lower cost.